If you restored Office from a backup, or you deleted and reinstalled your Office apps and you had purchased an Office 365 subscription through the Mac App Store, you might be able to restore the items you've previously paid for.
Sign out of the Office app through which you made the In-App purchase. To find out which Office app you used to make the purchase, see below.
Close and re-open the app.
Click Restore previous purchase at the bottom of the Get started with Office 365 window.
Obtain
Find out which app you used to make the purchase
2018-4-16 You are repeatedly prompted to enter your product key when you try to start any Office for Mac 2011 application. You are prompted to enter the product key and activate the program or to purchase the product online. You receive this prompt even though the product is licensed. You copy all Microsoft Office for Mac 2011 folders and files. Office 2019 is a one-time purchase that comes with classic apps like Word, Excel, and PowerPoint for PC or Mac, and does not include any of the services that come with an Office 365 subscription. One-time purchases don’t have an upgrade option, which means if you plan to upgrade to the next major release, you'll have to buy it at full price.
Microsoft Office For Mac Product Key
Use the Mac App Store app to review your subscriptions.
Sign in and look for a subscription starting with Office 365 under Active Subscriptions. For detailed instructions, see How to change or cancel subscriptions on your Mac.
The app icon determines which application you used to purchase the subscription.